The present invention relates to methods and apparatus for removing a gas from a liquid, and more particularly relates, in one embodiment, to methods and apparatus for separating oxygen from water, particularly on an offshore hydrocarbon recovery platform.
In many industries, including oil, paper and pulp, textile, electricity generating and food processing, there is an ever-present problem of handling water contaminated with various substances. In particular, water is often used to aid in the production of oil and gas on offshore platforms. This water is usually pumped into a formation in order to be able to pump oil out. The water pumped into the formation is typically available water, and is sea water in the case of offshore platforms. Seawater, like all naturally available water, contains small concentrations of oxygen, typically on the order of 6-10 ppm. The pumps, pipes and other structures through which the sea water is passed prior to injection into a subterranean formation typically are iron or copper alloys. The corrosion of these metals is catalyzed by the small quantities of oxygen present in the sea water, and thus it is desirable to remove as much of this oxygen as possible prior to transporting the water through the pipes, pumps, and other apparatus prior to formation injection. Because it is especially difficult to replace and repair equipment in offshore drilling operations due to much of the equipment being underwater and relatively inaccessible, it is particularly important to minimize corrosion of the equipment as much as possible.
Current practices for removing oxygen from water include stripping towers employing natural gas, and/or using a vacuum to reduce the boiling pressure of the water. Such prior art techniques usually cannot remove the oxygen to trace levels and thus chemical scavengers such as sulfites are used to remove oxygen further in a separate step. Unfortunately, floating offshore platforms typically are not tolerant of excessive vessel heights that are required of conventional stripping towers for oxygen removal. It would thus be advantageous to discover a method and apparatus for removing oxygen from water in an efficient manner involving a shorter physical profile, and particularly in an apparatus that is adapted for use on an offshore floating platform.
Apparatus for ingesting and mixing gas into a liquid body are known, such as those of U.S. Pat. No. 3,993,563, that includes a tank, a rotatable impeller fixed to a vertical drive shaft, and a vertically-extending conduit which surrounds the drive shaft and which extends to location in the liquid above the impeller to serve as a channel of communication between a source of gas and the impeller.
Accordingly, it is an object of the present invention to provide an apparatus for displacing a gas from a liquid, which apparatus is particularly suited to be used on floating offshore hydrocarbon recovery platforms.
It is another object of the present invention to provide a mechanical, cylindrical gas scavenger machine having a reduced height as compared with a stripping tower with a sump for chemical scavenger treatment to reduce oxygen content in a fluid such as water.
In carrying out these and other objects of the invention, there is provided, in one form, an apparatus for removing a gas from a liquid, where the apparatus includes a vessel for receiving a flow of liquid having a first gas contained therein, and where the vessel has a plurality of partitions sequentially dividing the vessel into at least a first gasification chamber and a second gasification chamber. Each adjacent chamber is in fluid or liquid communication with one another. Each chamber also has a vapor space, and there is no communication between the vapor spaces of adjacent chambers. The vessel also includes an inlet to introduce the flow of liquid into the gasification chambers. There is present a mechanism for ingesting and mixing a second gas into the liquid of each gasification chamber for creating a turbulent area and for displacing at least a portion of the first gas to the vapor space of each chamber. Finally, there is a vent in each chamber of the vessel for removing gas plus a liquid outlet from the outlet chamber.